Note 1. The 85 percentile speed is the speed which 85% of drivers do not exceed. It is not the average or mean speed of traffic, which will be considerably lower. The 85 percentile speed is often used by the Highway authority in setting speed limits or determining whether traffic calming is required. Provided the 85 percentile speed is below the enforceable speed limit (ie the signed speed limit +10% + 2mph), normally no action is taken. If the 85 percentile speed is consistently exceeded, the Highway authority may give consideration to traffic calming measures or raising the speed limit.
Note 2. Average weekly vehicle speeds by hour can be heavily distorted late at night or during the early hours, due to the low number of vehicles (in some cases, no vehicles at all). For example, if only one vehicle passes between 2am and 3am, then whatever speed that vehicle was doing will be the average. If two vehicles pass, one at 50mph and the other at 10mph, the average will be 30mph. If no vehicles pass, then the average speed will be zero.
Note that individual vehicle types or registration numbers are not recorded. We do not know whether an isolated vehicle recorded travelling at high speed was a police car on an emergency call, someone taking a sick person to hospital, or just a reckless driver.